1. Field of the Invention
The present invention relates to the fixing device for an image-forming device, and in particular, to the fixing device for an electrophotographic apparatus where induction heating is employed as a heating source.
2. Description of the Related Art
According to the conventional fixing device of an electrophotographic apparatus, the fixing is generally performed as follows. Namely, a halogen lamp is employed as a heating source, wherein the halogen lamp is disposed inside a metal roller to heat the roller, and an elastic roller is pressed onto the metal roller, thereby press-contacting a fixable material (a material to be fixed) onto the metal roller. These rollers are then rotated and the fixable material is passed through an interface between these rollers. Alternatively, a method of heating the roller in a non-contact manner by making use of a flash lamp is also put to practical use.
According to the conventional fixing device of this system however, since the heating roller is heated by making use of a lamp, the heat efficiency thereof is at most about 70%. Additionally, since the heating roller is constructed such that it is heated from the inside thereof, the heating roller is accompanied with various problems that the temperature-rise characteristic is poor, that the structure thereof is rather complicated, and that it is difficult to miniaturize the heating roller.
With a view to improve the efficiency of the fixing device, there have been proposed a fixing device which is designed to be heated through the utilization of induction heating, wherein an exciting coil is disposed inside the heating roller, and high-frequency current is applied to the exciting coil to thereby heat the heating roller through the generation of eddy currents in the heating roller.
In the case of the fixing device utilizing the induction heating as mentioned above, it is desired that a stable and effective heating thereof is required to be achieved. However, there still remain various problems that should be sufficiently studied, particularly with respect to the kinds of parameters that must be taken into account as well as with respect to the kinds of apparatus to be employed for that purpose.
Additionally, it is generally required, in order to obtain a predetermined quantity of heat, to employ a transformer for amplifying the current to be fed to the exciting coil. However, the employment of such a transformer would lead to an increase in the manufacturing cost of the fixing device.
Further, it is stipulated by regulations that the magnitude of the electric field leak should be controlled to be not more than a prescribed value for the sake of safety. Therefore, it is now required to find out parameters to meet the stipulated regulations.
Therefore, it is an object of the present invention to provide a fixing device which makes it possible to perform a stable and effective heating, which can be cheaply manufactured, and which is capable of easily meeting the requirement stipulated by regulations with regard to the magnitude of the electric field leak.
According to the present invention, there is provided a fixing device comprising an endless heating member which is adapted to be contacted with a fixable member to thereby enable an image on the fixable member to be thermally fixed; an electromagnetic inductive coil member disposed close to the endless heating member and inductively heating the endless heating member; and an AC power supply circuit supplying an AC current to the electromagnetic inductive coil member; wherein the coil member is constructed such that in a state where the coil member is positioned in the fixing device, a ratio between an inductance L of the coil member and a load resistance R of the coil member meets a condition of: L/R less than 50xc3x9710xe2x88x926 (H/xcexa9)
According to the present invention, there is provided a fixing device comprising an endless heating member which is adapted to be contacted with a fixable member to thereby enable an image on the fixable member to be thermally fixed; an electromagnetic inductive coil member disposed close to the endless heating member and inductively heating the endless heating member; and an inverter driving circuit supplying an AC current to the electromagnetic inductive coil member; wherein the coil member is constructed such that in a state where the coil member is positioned in the fixing device, a ratio between an inductance L of the coil member and a load resistance R of the coil member meets a formula of: L/R less than 50xc3x9710xe2x88x926 (H/xcexa9), and the inductance L and the load resistance R meet a formula of: V2/[(2xcfx80fL)2+R2]xe2x88x92xc2xd greater than 600 under conditions where a supply voltage of the inverter driving circuit is in the range of 100 to 250V, and a frequency thereof is in the range of 20 to 50 kHz.
It is preferable in this fixing device that the coil member is constructed such that an electric current I which is represented by a formula of: I=(V/R)(1xe2x88x92exe2x88x921/f/L/R) meets a withstanding current of the switching element of the inverter driving circuit under the conditions where a supply voltage of the inverter driving circuit is in the range of 100 to 250V, and a frequency thereof is in the range of 20 to 50 kHz.
The fixing device which is constructed as described above may be provided with the following specific features.
1. The thickness of the endless heating member is in the range of 0.1 to 10 mm.
2. The endless heating member is cylindrical having a diameter ranging from 20 to 60 mm.
3. A gap between the endless heating member and the coil member is in the range of 1 to 4 mm.
4. The frequency of the AC current is in the range of 20 to 50 kHz.
5. The material comprised in the endless heating member has a relative magnetic permeability of not more than 200.
6. The material comprised in the endless heating member is selected from the group consisting of iron, stainless steel, aluminum and a composite material comprising stainless steal and aluminum.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.